Method for rapid manufacture of nitrided thick layer of super high hardness on ferrous metal articles

ABSTRACT

A METHOD FOR THE RAPID MANUFACTURE OF A THICK NITRIDED LAYER OF SUPER HIGH HARDNESS ON FERROUS METAL ARTICLES WHICH COMPRISES THE STEPS OF PROVIDING A RIPENED FUSED SALT BATH IN A VESSEL, SAID SALT BOTH BEING PRODUCED BY ADDING A CHLORIDE TO A CYANIDE COMPOUND OR MIXTURE OF CYANIDE COMPOUNDS AND REPEATING THE CYCLE SUCCESSIVELY HEATING SAID BATH TO THE BOILING POINT THEREOF AND COOLING IT TO ROOM TEMPERATURE SEVERAL TIMES, AND TREATING SAID FERROUS METAL ARTICLE WITH SAID RIPENDED FUSED BATH. IN A PREFRRED EMBODIMENT, THE FUSED BATH CONTAINS A CATALYST COMPRISING A COMPOUND OF TITANIUM OR ZIRCONIUM.

United States Patent 3,639,641 METHOD FOR RAPID MANUFACTURE OF NITRIDED THICK LAYER 0F SUPER HIGH HARDNESS 0N FERROUS METAL ARTICLES Shinzoh Satoh, 548 Kichishoji, Musashino-shi, Tokyo, Japan No Drawing. Continuation-impart of application Ser. No. 573,499, Aug. 19, 1966. This application Mar. 20, 1969, Ser. No. 810,441

Claims priority, application Japan, Aug. 31, 1965,

Int. Cl. C23c 9/14 US. Cl. 148-155 6 Claims ABSTRACT OF THE DISCLOSURE A method for the rapid manufacture of a thick nitrided layer of super high hardness on ferrous metal articles which comprises the steps of providing a ripened fused salt bath in a vessel, said salt bath being produced by adding a chloride to a cyanide compound or mixture of cyanide compounds and repeating the cycle of successively heating said bath to the boiling point thereof and cooling it to room temperature several times, and treating said ferrous metal article with said ripened fused bath. In a prefrred embodiment, the fused bath contains a catalyst comprising a compound of titanium or zirconium.

Thisapplication is a continuation-in-part of application Ser. No. 573,499, filed Aug. 19, 1966 and now abandoned.

This invention concerns a method for rapid nitriding by which very superior effect is obtained, a hardened layer over 0.2 mm. in thickness is formed on the surface of the treated articles in only 2 hours, the maximum hardness of the hardened layer when applied to mild steel is Vickers hardness 1,.l001,200, this hardened layer does not spall off easily because it adheres to the substrate very strongly, the impact resistance is very high, it has very good resistance against fatigue and dimensional stability before and after treatment (expansion) is very small. The diffusion of nitrogen extends very deeply, i.e. from 2 mm. to 3 mm., the strength of the part in which nitrogen has diffused is much greater than that of the original material so that low quality material can be used in place of high quality material which makes possible reduction in the material cost as well as processing cost.

The basis of rapid nitriding by which such remarkable effects can be obtained is the salt bath and the presence of a special metal in the bath as a strong catalyst which accelerates nitriding. The method of preparing this salt bath is next explained.

NaCN or KCN or a mixture of both is the main component of the salt bath used, to this is added below 10% of KCl, NaCl or a halide of other metals. This is melted by heating, the temperature raised further to the boiling point, maintained at the boiling point for 2 to 3 minutes, cooled rapidly to below the melting point, heated again rapidly to the melting point, maintained at this temperature for 2 to 3 minutes and cooled rapidly to solidify. This heating and cooling cycle is repeated more than 5 times, by which the content of CNO compound in the salt bath is increased to about 4%. As a result of this operation, the quantity of production of carbonates which are not required for nitriding is kept to the minimum and the quantity of production of CNO compound which can be used effectively is increased up to the required quantity. At the same time, this operation reduces substantially wasteful decomposition and scattering of the CN compound. The necessary quantity of CNO compound formed by this operation is only 45% as compared with a Patented Feb. 1, 1972 minimum of 20% by the conventional, so-called soft nitriding process.

The next feature of this method is that a special metal having a strong catalytic effect is made to be present in the salt bath having the above composition. In the method of US. Pat. No. 3,074,860, activated pure titanium manufactured by electrolysis of an aqueous solution of a watersoluble compound is made to be present, in which electroplated articles are dipped several times and in a considerable quantity into a salt bath of 550-600 C. until the desired content is obtained. This electrolytically precipitated activated pure titanium gasifies readily at above 550 C. and is contained in the salt bath. It is possible to have the salt bath contain the electrolytic titanium by charging the above electrolytic titanium into the salt bath and removing it after 15 to 20 minutes. The necessary quantity in the salt bath at this time is over p.p.m. but the content from the point of view of operation is 250-350 p.p.m. and a maximum of 400 p.p.m. is sufficient. The catalytic effect is weak when this is below 100 p.p.m. and consequently, cannot be used.

'Electro-plated zirconium metal which has been manufactured by the same method as titanium can also be used in this method in a similar manner as titanium. Also, electrolytic titanium can be used together with electrolytic zirconium and in either case, the necessary quantity is below 400 p.p.m.

The reason Why a thick nitrided layer can be obtained with this bath containing a small quantity of CNO compound according to this process is explained next.

In nitriding by the soft nitriding method, nitrogen in the nascent state is formed by decomposition of the necessary quantity of CNO compound and this controls nitriding.

In the present process, however, nitrogen in the nascent state is formed by the bonding reaction with oxygen due to the strong catalytic effect of activated titanium or activated zirconium as follows:

The quantity of NaCNO required is about A to /s of Eq. A or Eq. B. Furthermore, as the bonding reacting with oxygen progresses easily, it is not necessary to form NaCNO by blowing in air or oxygen which is carried out in the soft nitriding method. Consequently, the operation of preparing the necessary salt bath composition is easy and furthermore, the management of the operation of the bath is very easy.

The nitriding ability of this bath is very good and it is not necessary to remove oil and fats which adhere to the steel products but they can be dipped directly into the salt bath, maintained for the necessary time and then water-cooled, which makes the operation easy. It can be said that this is an advanced process which improves further US. Pat. No. 3,194,694.

Furthermore, steel material having as-rolled surface, that is the so-called black sheet (oxidized film) can be nitrided as it is by dipping into the bath, also the cut surface which has been cut with oxy-acetylene flame can be nitrided without cleaning. Or, the surface which has been electric welded or gas welded can similarly be nitrided without cleaning by dipping as it is directly into the nitriding salt bath.

The nitrided hard layer formed by this process has a matrix in which cementite is finely and uniformly dispersed in u-iron and a metal composition containing nitrided compound, Fe N. It is therefore clear that it is not brittle and has toughness in spite of the high Vickers hardness of 1,100-l,200, it has high impact strength and high rea 3 sistance against fatigue. Particularly, there is no danger of spalling off in case of sharp edge such as a cutter blade and there are examples of durability of 3 times the conventional blades.

Nitriding according to this method can be carried out very easily as it is only necessary to dip the specified steel product for a standard time of 2 hours (in case of mild steel) in a bath heated to 600 C., removing this from the bath and cooling with water. The management of the bath is easy, as, for example, it is only necessary to add NaCN below 3% of the total bath quantity before operation is started, dip the workpiece into this and maintain for the specified time, remove and wash this with water. It is possible to use the same salt bath for over 1 months without adding fresh salt bath agents even when used continuously for 8 hours daily.

EXAMPLE 1 50 kg. of a mixture composed of 90% NaCN and 10% KCl (weight percent) is melted by heating, this is heated further rapidly up to the boiling point, maintained at the boiling point for 23 minutes, quick-cooled to solidify and heated again to boil, maintained for 2-3 minutes and quick-cooled to solidify. This cycle is repeated more than times, then the temperature of the salt bath is maintained at 600 C., numerous iron products having pure titanium metal which has been electroplated by the water-soluble electrolytic process (US. Pat. No. 3,074,860) are charged into this and then removed. This operation is carried out several times to prepare a bath containing 300 p.p.m. titanium. This bath is heated and maintained at 600 C., a steel article formed from 3.2 mm. thick mild steel sheet containing 0.2% carbon is dipped in this for 2 hours and then water-cooled, by which the hardened layer becomes a thickness of 0.25 mm. from the surface and the maximum hardness was Vickers 1,1 80.

EXAMPLE 2 '5 cycles of heating and cooling was in Example 1 was carried out with 50 kg. of a mixture composed of 45% NaCN, 45% KCN and KCl (all in weight percent), activated titanium and activated zirconium are made to be present simultaneously in the salt bath in the same manner as in Example 1 so as to prepare a salt bath with a total content of these 280 p.p.m., this was heated to 600 C., a suitable carburized gear containing 0.18% carbon was dipped in this for 2 hours and then water-cooled, by which the hardened layer was 0.3 mm. in thickness and maximum Vickers hardness was 1,120.

EXAMPLE 3 5 cycles of heating and cooling as in Example 1 was carried out with 50 kg. of salt bath composed of 60% NaCN, KCN and 10% KCl (all in weight percent), activated titanium prepared in the same manner as in Example 1 was made to be present in the salt bath and its total content was 250 ppm. This bath was heated to 600 C., a carbon steel shaft (diameter mm., length 220 mm.) containing 0.35% carbon was dipped in this for 2 hours, removed and water-cooled, by which the hardened layer was 0.23 mm. in thickness and the maximum Vickers hardness was 1, 1 50.

What I claim is:

1. A method for the rapid manufacture of a nitrided thick layer of super high hardness on ferrous metal articles which comprises treating the articles in a salt bath containing a catalyst selected from the group consisting of electrolytically activated titanium, electrolytically activated zirconium and mixtures thereof in an amount of ppm. to 400 ppm. at a temperature of 600-750 C., until the desired nitridiug effect is produced and then removing the nitrided product, said bath being prepared by melting a salt bath whose principle components are cyanides, heating the bath rapidly up to its boiling point, maintaining the bath at this temperature for 2-3 minutes, repeating this heating and cooling operation for more than about 5 times so as to produce about 4% C-NO in the bath.

2. A method according to claim 1 wherein the catalyst is titanium, said catalyst being introduced into the bath by placing an article plated with pure titanium into the bath, which bath has been heated to 560600 C., maintaining the plated titanium article in the bath at this temperature for 20 minutes to bring the content of the titanium in the salt bath up to between 100 ppm. and 400 p.p.m. and then removing the titanium plated article therefrom.

3. A method according to claim 1 wherein the catalyst is Zirconium dispersed throughout the salt bath.

4. A method according to claim 1 wherein the salt bath contains a mixture of zirconium and titanium dispersed throughout said bath.

5. A method according to claim 1 wherein iron or steel articles are completely nitrided by dipping the articles into the bath without the necessity of previously cleaning the surface of said articles and then removing the iron or steel articles and quickly cooling.

6. A method according to claim 1 wherein the cyanide is selected from the group consisting of NaCN, KCN and mixtures thereof.

References Cited UNITED STATES PATENTS 2,334,723 11/1943 Neumark 23-75 3,022,204 2/1'962 Muller et a1. 14815.5

3,194,694 7/1965 Satoh 148l5.5

3,208,885 9/1965 Muller et al. 148-l5.5

FOREIGN PATENTS 595,558 12/1947 Great Britain 14815.5

810,476 3/1959 Great Britain 14828 OTHER REFERENCES Molten Salt Baths for Heat Treatment and Case Hardening of Steel, DuPont Co. Inc., Wilmington, Del., 1942, relied on pp. 5-9, 23 and 24.

Transactions of ASM, vol. 27, 1939, pp. 935942.

The Iron Age, Jan. 20, 1966, pp. 64 and 65.

CHARLES N. LOVELL, Primary Examiner US Cl. X.R. 14S28 

